Forget Code, Bathrooms Need Fans.

For the last 800 years, building codes have allowed bathrooms to be built without exhaust fans.

Exhaust fans aren’t even a requirement here in Minnesota. This is a great example of how building codes are only minimum standards. I thought about this while doing a home inspection at a rental home in Minneapolis. The outdoor temperature was about 45 degrees, and every single window in the home was covered with condensation, which was also dripping down the walls.

Oh, and there were no fans installed.

Bathrooms need exhaust fans to help prevent moisture problems, plain and simple. When people take showers and baths, moisture gets pumped in to the air. During the winter, this moisture condenses on windows and walls, and often makes it’s way in to the attic space through attic bypasses, where it will create frost.

Minnesota requires windows in bathrooms that provide a total glazed area of at least three square feet, and half of that must be openable (R303.3). The exception to this rule comes when a bath fan is installed that will exhaust at least 50 cubic feet per minute, or a continuous exhaust system such as a Heat Recovery Ventilator that exhausts at least 20 cubic feet per minute.

The idea of someone actually opening a window on a cold winter day in Minnesota to help reduce moisture in the bathroom is pretty far fetched. If you live in a house without an exhaust fan in a bathroom that gets used for showers or baths, you would do well to install one. Your house will thank you for it.

If you’re going to install a fan, here are a few tips to make sure your house is happy with the fan.

Choose a good fan. You’ll want to balance noise level, performance, and price. If you buy a cheap noisy fan, you probably won’t even want to turn it on. On the other hand, if you’re installing the fan in a half bath near the living spaces, you might want to install a noisy fan on purpose…

Make the exhaust duct short. A proper exhaust duct will be as short as possible and take as few turns as possible. The longer the duct and the more twists and turns it takes, the less air flow. A fan rated for 80 cubic feet per minute (CFM) assumes the fan has no duct. As soon as a duct gets added, the actual CFM goes down. I’ve inspected hundreds of houses where there is barely any air flow at bath fan exhausts. If the bath fan is located in the basement and the duct runs up to the roof at the second story, air flow will be pretty pathetic.

Insulate the duct where it passes through unconditioned spaces, such as the attic. If you don’t, moisture will condense it the duct, and might drip down and stain the ceiling. I once inspected a house in Richfield where the exhaust duct was uninsulated in the attic, and so much moisture had accumulated in the duct that it was completely filled with water. The photo at right shows me holding my flashlight up against the duct – this is one of my favorites. Click the thumbnail to see the full version.

Don’t use a standard switch to control the fan. When a single switch controls the fan, the fan usually doesn’t end up running long enough to remove enough moisture. A better solution would be to install a timer that runs for at least a half hour, or a humidity sensing switch.

About the Author

Reuben

Reuben is a second generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben has worked for Structure Tech since it was purchased by Neil in 1997, and is now co-owner and President of the company.

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13 Comments

D Graham 1 YEAR AGO

@D Graham - put the fan at the ceiling in the middle of the room. There is no required distance from a shower head.

Mark 2 YEARS AGO

Is it better to run duct to ridge vent exit points along roof ridge and have shorter duct or run longer duct to vertical wall (unheated attic window or gable vent)? The duct would be 4" solid surface in either case. Ridge vent run would be about 12 feet + elbow; vertical run would be about 25 feet plus elbow.
Thank you.

The fan should definitely not exhaust to the ridge vent. Run the exhaust to the gable end.

Steve 3 YEARS AGO

I have two adjacent bathrooms, one with a fan and one without. If I install another bath fan, can they share the existing vent through the roof? I'm hesitant to poke another hole through the roof. Is a soffit vent a better option?

Hi Steve,
No, the fans should not share the same terminal. This will reduce the performance of the fans, and will allow for one fan to blow some air back in to the other bathroom when both fans aren't running at the same time. The fans also shouldn't be venting through the soffit - you probably have soffit vents that would bring this moist air back in to the attic. If you have a gable end wall in the attic, that would be a good option. Otherwise, don't worry about making a hole in your roof for the vent cap. It's really not a big deal.

Janie Jurgens 5 YEARS AGO

Hello,
I appreciate the information you have provided, thanks for sharing. I have a few questions that weren't answered by your blog and am wondering if it's something you can touch on. If not, can you recommend some resources?
Do you think exhausting the fan horizontally with a minor downward slope out an exterior wall is better than vertical through the attic and out the roof? Are there any draw backs? How far away from exterior windows should the exhaust vent be to prevent the warm, moist air from rotting the wood? What are you thoughts on a wall vs. ceiling mount fan? Is it better to place a ceiling mount fan above the shower, toilet or in the center of the bathroom?
Regards,
Janie Jurgens

Janie - great questions, and I wish I knew the best answers! I don't think exhausting the fan horizontally or vertically would make much of a difference, and I don't know of any drawbacks to either of those installations. I'd go with whatever would give me the shortest duct length.
The Minnesota Mechanical Code requires bath fans to terminate at least three feet from doors, operable windows, and nonmechanical intake openings (Minnesota Mechanical Code section 401.5.2). As long as the wood at the exterior has a good coat of paint, you shouldn't have to worry about it rotting.
I think that a ceiling mounted fan is preferable... but I really don't know why. I have no good evidence to back up my preference. I've always placed ceiling fans just outside the shower area. I think that placing the fan inside the shower area would be best, but some manufacturers require the fan to be GFCI protected when this is done, so I've never bothered with that. The main goal of the fan is to get rid of moisture, so put the fan where it will be most effective at doing that.

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